Lifespan Development. Tara L. Kuther

Persaud, 2016). After entering the female’s vagina, the sperm’s tail propels it through the cervix into the uterus and onward toward the ovum. Sperm must travel for about 6 hours to reach the fallopian tube, where an ovum may—or may not—be present. The journey is difficult: Some sperm get tangled up with other sperm, some travel up the wrong fallopian tube, and others do not swim vigorously enough to reach the ovum. On average, about 300 sperm reach the ovum, if one is present (Webster, Morris, & Kevelighan, 2018). Those that travel up the fallopian tube can live up to 6 days, able to fertilize a yet unreleased ovum. The ovum, however, remains viable for about only a day after being released into the fallopian tube.

      Description

      Figure 3.1 Female Reproductive System

      Source: Levine and Munsch (2010, p. 102).

      Both sperm and the woman’s reproductive tract play a role in fertilization (Suarez, 2016). Sperm are guided by temperature, tracking the heat of an expectant ovum, as well as by chemical signal (Lottero-Leconte, Isidro Alonso, Castellano, & Perez Martinez, 2017). In the presence of an ovum, sperm become hyperactivated, they swim even more vigorously, and the sperm’s head releases enzymes to help it penetrate the protective layers of the ovum (Bianchi & Wright, 2016). As soon as one sperm penetrates the ovum, a chemical reaction makes the ovum’s membrane impermeable to other sperm. The sperm’s tail falls off, and the genetic contents merge with that of the ovum.

      At the moment of conception, the zygote contains 46 chromosomes, half from the ovum and half from the sperm. After fertilization, the zygote rapidly transforms into a multicelled organism. Prenatal development takes place over three developmental periods: (1) the germinal period, (2) the embryonic period, and (3) the fetal period.

      Germinal Period (0 to 2 Weeks)

      During the germinal period, also known as the period of the zygote, the newly created zygote begins cell division as it travels down the fallopian tube, where fertilization took place, toward the uterus. About 30 hours after conception, the zygote then splits down the middle, forming two identical cells (Webster et al., 2018). This process is called cleavage, and it continues at a rapid pace. As shown in Figure 3.2, the two cells each split to form four cells, then eight, and so on. Each of the resulting cells is identical until about the third set of cell divisions. This process of cell division continues rapidly. Any of these cells may become a person (and sometimes do, in the case of monozygotic or identical twins).

      This ball of cells, known as a morula, is formed at about three days after conception. Each of these cells is identical. Differentiation has not yet begun.

      Pascal Goetgheluck / Science Source

      Cell differentiation begins roughly 72 hours after fertilization when the organism consists of about 16 to 32 cells. Differentiation means that the cells begin to specialize and are no longer identical. By 4 days, the organism consists of about 60 to 70 cells formed into a hollow ball called a blastocyst, a fluid-filled sphere with cells forming a protective circle around an inner cluster of cells from which the embryo will develop.

      Development proceeds very quickly during the embryonic period. Note the dramatic changes from the fifth week (left) to the seventh week (right) of prenatal development.

      Wikimedia / Petit Format / Science Source

      Description

      Figure 3.2 Germinal Period

      Source: Levine and Munsch (2010, p. 102).

      Implantation, in which the blastocyst burrows into the wall of the uterus, begins at about day 6 and is complete by about day 11 (K. L. Moore & Persaud, 2016). By the end of the second week, when fully implanted into the uterine wall, the outer layer of the blastocyst begins to develop into part of the placenta, the principal organ of exchange between the mother and developing organism. The placenta will enable the exchange of nutrients, oxygen, and wastes via the umbilical cord. Also, during this stage, the developing organism is encased in amniotic fluid, providing temperature regulation, cushioning, and protection from shocks.

      Embryonic Period (3 to 8 Weeks)

      By the third week after conception, the developing organism—now called an embryo—begins a period of structural development during which the most rapid developments of the prenatal period take place. All of the organs and major body systems form during this embryonic period. The mass of cells composing the embryonic disk develops into layers, which will develop into all of the major organs of the body. The ectoderm, the upper layer, will become skin, nails, hair, teeth, sensory organs, and the nervous system. The endoderm, the lower layer, will become the digestive system, liver, lungs, pancreas, salivary glands, and respiratory system. The middle layer, the mesoderm, forms later and will become muscles, skeleton, circulatory system, and internal organs.

      During the third week, at about 22 days after conception, the endoderm folds to form the neural tube, which will develop into the central nervous system (brain and spinal cord) (Webster et al., 2018). Now the head can be distinguished. A blood vessel that will become the heart begins to pulse and blood begins to circulate throughout the body. During days 26 and 27, arm buds appear, followed by leg buds on days 28 through 30. At about this time, a tail-like appendage extends from the spine, disappearing at about 55 days after conception (Sadler, 2015). The brain develops rapidly and the head grows faster than the other parts of the body during the fifth week of development. The eyes, ears, nose, and mouth begin to form during the sixth week. Upper arms, forearms, palms, legs, and feet appear. The embryo shows reflex responses to touch.

      During the seventh week, webbed fingers and toes are apparent; they separate completely by the end of the eighth week. A ridge called the indifferent gonad appears; it will develop into the male or female genitals, depending on the fetus’s sex chromosomes (K. L. Moore & Persaud, 2016). The Y chromosome of the male embryo instructs it to secrete testosterone, causing the indifferent gonad to create testes. In female embryos, no testosterone is released, and the indifferent gonad produces ovaries. The sex organs take several weeks to develop. The external genital organs are not apparent until about 12 weeks.

      At the end of the embryonic period, 8 weeks after conception, the embryo weighs about one seventh of an ounce and is 1 inch long. All of the basic organs and body parts have formed in a very rudimentary way. The embryo displays spontaneous reflexive movements, but it is still too small for the movements to be felt by the mother (Hepper, 2015). Serious defects that emerge during the embryonic period often cause a miscarriage, or spontaneous abortion (loss of the fetus); indeed, most miscarriages are the result of chromosomal abnormalities. The most severely defective organisms do not survive beyond the first trimester, or third month of pregnancy. It is estimated that up to 45% of all conceptions abort spontaneously, and most occur before the pregnancy is detected (Bienstock, Fox, & Wallach, 2015).

      Fetal Period (9 Weeks to Birth)

      During the fetal period, from the ninth week to birth, the organism, called a fetus, grows rapidly, and its organs become more complex and begin to function. The end of the third month marks the close of the first trimester, at which time all parts of the fetus’s body can move spontaneously, the legs kick, and the fetus can suck its